Influenza virus infection is a major cause of excess mortality in individuals above 65 years of age. When influenza vaccine is given to the elderly, and when it is immunogenic, a significant reduction in excess mortality occurs. The immune response in elderly, however, is variable for certain virus strains and for the vaccine in general. Interest to determine when protective levels of antibody first appear following influenza vaccination, to help guide in determining how to use vaccine and anti-flu virus drug during an epidemic.